The present invention generally relates to the production of composite glass-plastic articles, and is particularly concerned with a process for the manufacture of optically clear composites of laminated structure for a variety of applications.
The advantages of combining the light weight of clear plastics with the scratch resistance and chemical durability of glass have long been recognized. U.S. Pat. No. 2,361,589 to Bennett et al., for example, describes durable, lightweight lenses formed by laminating thin glass sheets to the exterior of plastic lenses.
Although composite products exhibiting the desired physical properties can be produced by lamination methods, the direct molding or casting of such composites is more efficient and thus more attractive from the commercial point of view. One procedure for adapting such direct methods to the manufacture of optical composites is described by Coenen in German Patentschrift 1,529,861.
Among the optically clear plastics suitable for use in composites of this type are certain thermosetting plastics exhibiting high shrinkage on curing. Examples of such plastics are epoxy and allyl carbonate plastics. As suggested in U.S. Pat. No. 3,382,137 to Schreiber et al., curing shrinkage such as is exhibited by plastics of this type can generate very high stresses in direct-cast glass-plastic composite articles.
The minimization of such stresses can be important for optical applications. For example, in the case of the plastic-plastic polarizing lenses described in U.S. Pat. No. 3,970,362 to Laliberte, excess stress was found to introduce birefringence into the lenses. Careful positioning of the polarizing plastic element within the optical plastic lens body was therefore specified in order to minimize the optical affects of any residual stress.
Stress is also a problem in the case of cast glass-plastic lenses having, for example, only a front surface protected by a thin glass element. In this configuration, shrinkage of the plastic during curing can cause deformation of the lens and cracking of the protective glass.
Some thermosetting plastics generate only moderate stress during shrinkage, and then fail due to plastic cracking prior to full curing. In cast glass-plastic composites produced from these plastics, locally adhering plastic areas bounded by a network of cracks in the plastic are usually produced. Of course such composites are useless for optical applications.
It is a principal object of the present invention to provide a direct casting method for producing composite glass-plastic articles wherein residual stresses attributable to shrinkage of the cast plastic are eliminated.
It is a further object of the invention to provide a direct casting method which can be used for the manufacture of glass-surfaced plastic lenses exhibiting low residual stress.
Other objects and advantages of the invention will become apparent from the following description.